//
// Created by spiro on 19-3-8.
//

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define LEN 40

int main(int argc, char * argv[]) {
    FILE *in, *out;
    int ch;
    char name[LEN];
    int count = 0;
    if (argc < 2) {
        fprintf(stderr, "Usage: %s filename\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    if ((in = fopen(argv[1], "r")) == NULL) {
        fprintf(stderr, "Can't open the file \"%s\"\n", argv[1]);
        exit(EXIT_FAILURE);
    }

    strncpy(name, argv[1], LEN - 5);
    name[LEN - 5] = '\0';
    strcat(name, ".red");
    if ((out = fopen(name, "w")) == NULL) {
        fprintf(stderr, "Can't create output file.\n");
        exit(3);
    }

    while ((ch = getc(in)) != EOF) {
        if (count++ % 3 == 0) {
            putc(ch, out);
        }
    }
    if (fclose(in) != 0) {
        fprintf(stderr, "Error in closing file: %s .\n", argv[1]);
    }
    if (fclose(out) != 0) {
        fprintf(stderr, "Error in closing file: %s .\n", name);
    }

    return 0;
}